1. Field of the Invention
The subject invention relates to the demodulation of time modulated signals and, more specifically, to pulse averaging demodulators for FM and other time modulated carriers.
2. Disclosure Statement
This disclosure statement is made pursuant to the duty of disclosure imposed by law and formulated in 37 CFR 1.56(a). No representation is hereby made that information thus disclosed in fact constitutes prior art inasmuch as 37 CFR 1.56(a) relies on a materiality concept which depends on uncertain and inevitably subjective elements of substantial likelihood and reasonableness, and inasmuch as a growing attitude appears to require citation of material which might lead to a discovery of pertinent material though not necessarily being of itself pertinent.
By way of background, reference may be had to my two prior U.S. Pat. Nos. 3,426,284, issued Feb. 4, 1969, and 3,783,398, issued Jan. 1, 1974, and herewith incorporated by reference herein. Both of these patents show pulse averaging demodulators for time modulated signals, employing four transistors grouped in two comparator pairs. A common threshold reference voltage is applied to one transistor in one comparator pair and to one transistor in the other comparator pair.
First and second repetitive pulses or ramps are generated in response to alternate successive zero crossings of the time modulated signal or carrier. The other transistor in one comparator pair is controlled with the first pulses or ramps and the other transistor in the other comparator pair with the second pulses or ramps, in an effort to generate a train of constant area pulses corresponding to zero crossings of the time modulated carrier. The information signal is reproduced by averaging constant area pulses in the latter train with the aid of suitable filters.
In the demodulator disclosed in the earlier of these patents, I employed a capacitor between the emitters of the two comparator pairs in order to cause each pulse or ramp controlled transistor, upon becoming conductive, to turn off all other transistors, whereby twice the output current variation for a given frequency change could be provided.
In practice, that approach required not only the provision and implementation of an extra capacitive component but also necessitated the use of two constant current sources or their functional equivalents.
In the system disclosed in my later patent, I no longer used the mentioned capacitor, but still provided an individual constant current source for each transistor pair. The peak-to-peak current of the square wave produced by that prior demodulator thus was equal to the current supplied or sinked by either of the two constant current sources. This compared to an average current of one and one-half the latter constant current, since there was always a conducting side among the two comparator pairs.
This yielded a maximum ratio of video signal current to standing current of only two-thirds at the demodulator output. This required a large amplification of the output signal to make up for the relatively low yield or efficiency of the demodulator. In turn, a special prefiltering stage was required to reduce ripple sufficiently to permit the requisite high output signal amplification.
This prior approach bears a certain similarity to my previous system, in which the above mentioned capacitor between comparator pairs reduced ripple problems caused by variations in collector currents of insufficiently matched transistors in the comparator circuits.